1. Field of the Invention
The present invention relates, in general, to electric cooking apparatus. More specifically, the invention relates to a cooking apparatus in which a cooking completion is determined by detecting the infrared rays from food to be cooked.
2. Description of the Prior Art
Generally, it is difficult to determine whether cooking is completed or not in the cooking operation, because it depends largely on a cook's intuition and experience. Recently, in electric cooking apparatus, such as, e.g., microwave ovens, automatic cooking has been provided. The temperature of food to be cooked is detected by a thermistor whose resistance value varies in response to the changes of the wave-length of the infrared rays radiated from the food.
The output of the thermistor representing the temperature of the food is compared with a predetermined temperature value, and the cooking completion is thereby determined.
An example of the above-described microwave oven is disclosed in Japanese patent application No. 54-31485 (Patent publication No. 28117/1985) filed Mar. 16, 1979, and entitled HIGH FREQUENCY HEATING APPARATUS. In this prior art, a thermistor is used as the infrared ray detection element. Infrared rays radiated from the food are intermittently supplied to the thermistor by the operation of a chopper. The resistance value of the thermistor varies in response to the changes of the wave-length of the infrared rays, and an AC signal is obtained as an output of the thermistor. Based on this AC signal, the temperature of the food can be determined.
According to the above-described prior art, automatic cooking may be carried out. However, in this prior art, since the changes of terminal voltage of the thermistor are small, it is difficult to accurately detect the temperature of food to be cooked on the basis of only the output signal of the thermistor. Therefore, it is necessary to use, as shown in the prior art, a chopper mechanism, a chopper temperature detection circuit and a photocoupler for detecting the on-off timing of the chopper mechanism. Furthermore, since thermistors have, in general, a thermal time constant, the output level (AC signal) of the thermistor is low, and this low output level often causes errors in the detection under the influence of foreign noise.
As shown in FIG. 1, elimination of these components, e.g., chopper, photocoupler, etc., from the prior art circuit, was considerd.
In FIG. 1, a first thermistor Th1 and a second thermistor Th2 are used for detecting temperatures. First thermistor Th1 receives infrared rays from the food to detect the temperature of the food. Second thermistor Th2 does not receive the infrared rays from food, but detects the actual temperature in the atmosphere surrounding these thermistors Th1 and Th2. First thermistor Th1 is grounded through a resistor r1, and second thermistor Th2 also is grounded through a resistor r2. A DC voltage (+Vdd) is supplied to the first and second thermistors Th1 and Th2. The outputs from the connecting points between first thermistor Th1 and resistor r1, and second thermistor Th2 and resistor r2, are input to amplifying circuit Am. A difference value between the output of first and second thermistors Th1 and Th2 is output from amplifying circuit Am. Therefore, the temperature of the food is determined on the basis of the difference value.
In the construction described above, however, since the properties of thermistors Th1 and Th2 generally differ from one another, errors may be included in the detected temperature. Therefore, the result of the cooking is not uniform. On the other hand, if thermistors having the same properties are used, the cost increases.